The subject matter disclosed herein relates to an electrochemical system for providing hydrogen.
Electrochemical hydrogen compression systems can compress hydrogen by electro-oxidizing it and subsequently electro-reducing the resulting protons in a high pressure chamber. In addition to compression of pure hydrogen, the process has been successfully demonstrated for removal of hydrogen from a mixture of hydrogen and helium, thus purifying the helium, as well as removal of hydrogen from other gas mixtures.
The throughput rate of hydrogen in an electrochemical separator stack or compressor is proportional to the electrical current at which the cell is run. The electrical current per unit active area is referred to as current density. Cell configurations with lower effective electrical resistance can be operated at higher current densities and are thus more efficient and cost effective. The bulk of the effective electrical resistance is contributed by the Ohmic resistance of the proton conducting membranes in the cells. The proton conductivity of this material is strongly influenced by its water content. Fully hydrated membranes are dramatically more conductive to protons than dry membranes.
In low pressure electrochemical purification cell stacks, there are highly effective means of maintaining the hydration of the membranes. This has been achieved by circulating liquid water on the product side of the cells. This imparts a two phase flow out of the cathode of the stack. This water/hydrogen mixture is sent to a phase separator stack and the liquid is cycled back to the cathode inlet. This circulating water loop may also be used for cooling purposes. This method is used in cells that operate with a hydrogen product pressure of up to around 200 psi.
It is not generally practical to use the same method of membrane hydration and system cooling in high product pressure configurations for a variety of reasons. It is not generally cost effective to include a pump with a suction side pressure rating high enough to circulate water that is pressurized to the system product pressure. Additionally, refilling the water chamber is a further system complication. Most critically, the water rapidly permeates from the cathode to the anode and floods the porous catalyst material and/or the generally hydrophilic material that supports the membrane. This flooding causes a loss of gas permeability and makes the stack effectively inoperable under certain conditions. As a consequence, under many circumstances, high pressure stacks must be operated with only gas humidification of their membranes, and are thus far more limited in current density than low pressure stacks.